Application of X-ray tomography to quantify macropore characteristics of loess soil under two perennial plants

With the advent of large-scale restoration of vegetation in the Loess Plateau, northwest China, there has been an increase in concern about the suitability of loess soil to support permanent vegetation cover. The quantification of soil macropore characteristics could be critical in determining the architecture and hydrological processes of loess soil on the plateau. In this research, we compared the effects of Purple alfalfa (Medicago sativa L.) and Korshinsk peashrub (Caragana korshinskii K.) on the macropore characteristics of a soil profile on the plateau with computed tomography (CT). To achieve this, undisturbed cores of soil were excavated from beneath purple alfalfa (ALF), 22-year-old Korshinsk peashrub (KOP22) and 40-year-old Korshinsk peashrub (KOP40) vegetation types in the Liudaogou watershed for evaluation. The soil macropore characteristics (including macroporosity, largest pore area, amounts of macropores, circularity, surface area density, branch density, junction density and connectivity) were determined with image analysis software. Soil under KOP22 and KOP40 treatments had approximately the same amounts ofmacropores (17 per 6359-mm(2) area), which was three times greater than those under ALF plants. Macroporosity ratios of soil under KOP22 and KOP40 plants to that under ALF plants were 2.3 and 3.6, respectively. Compared with KOP22, KOP40 had a larger macroporosity and the largest pore area at the 100-300-mm soil depth. The KOP plants, in particular KOP40, apparently improved the macropore network structure of the soil more than ALF. However, the macropores under ALF were much rounder at the 100-300-mm soil depth than those under the other two plants. There was no correlation between macropore characteristics and organic matter content of the soil at 100-400-mm depth. Nevertheless, macroporosity was strongly correlated with the largest pore area. The findings of this research are critical for developing strategies for the restoration of vegetation in the Loess Plateau through improvement of the hydrological process of loess soil.